Methods and apparatus for chemical-mechanical polishing utilizing low suspended solids polishing compositions

ABSTRACT

The present invention provides chemical-mechanical polishing (CMP) methods and apparatus suitable for polishing a substrate utilizing a low suspended solids slurry composition. The CMP methods of the invention comprise polishing a substrate with CMP slurry containing a low suspended solids level (e.g., about 0.01 percent by weight to about 1.0 percent by weight) of a particulate abrasive material in a CMP apparatus, while continuously monitoring and accurately maintaining a predetermined total suspended solids (TSS) level in the slurry. Preferably, maximum TSS variability of the slurry is less than about 20 percent (i.e., ±20% of the target TSS level), more preferably less than about 10 percent TSS variability (i.e., ±10% of the target TSS level) during the course of the polishing process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/201,001, filed on Dec. 5, 2008, which isincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to polishing methods and apparatus for polishinga substrate. More particularly, this invention relates tochemical-mechanical polishing methods and apparatus utilizing lowsuspended solids abrasive polishing compositions.

BACKGROUND OF THE INVENTION

Compositions and methods for chemical-mechanical polishing (CMP) thesurface of a substrate are well known in the art. Polishing compositions(also known as polishing slurries, CMP slurries, and CMP compositions)for polishing metal-containing surfaces of semiconductor substrates(e.g., integrated circuits) typically contain abrasives, variousadditive compounds, and the like, and frequently are used in combinationwith an oxidizing agent. Such CMP compositions are often designed forremoval of specific substrate materials such as metals (e.g., tungstenor copper), insulators (e.g., silicon dioxide, such as plasma-enhancedtertraethylorthosilicate (PETEOS)-derived silica), and semiconductivematerials (e.g., silicon or gallium arsenide).

In conventional CMP techniques, a substrate carrier (polishing head) ismounted on a carrier assembly and positioned in contact with a polishingpad in a CMP apparatus. The carrier assembly provides a controllablepressure (down force) to urge the substrate against the polishing pad.The pad and carrier, with its attached substrate, are moved relative toone another. The relative movement of the pad and substrate serves toabrade the surface of the substrate to remove a portion of the materialfrom the substrate surface, thereby polishing the substrate. Thepolishing of the substrate surface typically is further aided by thechemical activity of the polishing composition (e.g., by oxidizingagents present in the CMP composition) and/or the mechanical activity ofan abrasive suspended in the polishing composition. Typical abrasivematerials include, for example, silicon dioxide (silica), cerium oxide(ceria), aluminum oxide (alumina), zirconium oxide (zirconia), titaniumdioxide (titania), and tin oxide.

The abrasive desirably is suspended in the CMP composition as acolloidal dispersion, which preferably is colloidally stable. The term“colloid” refers to the suspension of abrasive particles in the liquidcarrier. “Colloidal stability” refers to the maintenance of thatsuspension during a selected period of time with minimal settling. Inthe context of this invention, an abrasive is considered colloidallystable if, when the abrasive is placed into a 100 mL graduated cylinderand allowed to stand without agitation for a period of time of about 2hours, the difference between the concentration of particles in thebottom 50 mL of the graduated cylinder ([B] in terms of g/mL) and theconcentration of particles in the top 50 mL of the graduated cylinder([T] in terms of g/mL) divided by the initial concentration of particlesin the abrasive composition ([C] in terms of g/mL) is less than or equalto about 0.5 (i.e., ([B]−[T])/[C]≦0.5). The value of ([B]−[T])/[C]desirably is less than or equal to about 0.3, and preferably is lessthan or equal to about 0.1.

U.S. Pat. No. 5,527,423 to Neville, et al., for example, describes amethod for chemically-mechanically polishing a metal layer by contactingthe surface of the metal layer with a polishing slurry comprising highpurity fine metal oxide particles suspended in an aqueous medium.Alternatively, the abrasive material may be incorporated into thepolishing pad. U.S. Pat. No. 5,489,233 to Cook et al. discloses the useof polishing pads having a surface texture or pattern, and U.S. Pat. No.5,958,794 to Bruxvoort et al. discloses a fixed abrasive polishing pad.

For some CMP compositions (e.g., ceria-based slurries) it often isdesirable to use a relatively low-solids dispersion (i.e., having anabrasive concentration at a total suspended solids (TSS) level of about1 percent by weight or less). For example, low-solids cerium oxidecompositions such as iDiel™ 6600 (Cabot Microelectronics Corp., Aurora,Ill.), which contains about 0.6 percent by weight cerium oxide, providesfor highly effective silicon dioxide (PETEOS) removal rates comparableto or exceeding silica-based slurries containing over 12 percent byweight of suspended abrasive, while simultaneously providing lowerpolishing defectivity than the silica slurries. We have recentlyrecognized that when such low-TSS slurries are utilized in a CMPprocess, slight absolute variations (e.g., 0.05 to 0.1 percent by weightvariability) in the TSS level of the slurry being delivered to thesubstrate can occur during polishing, which can have a dramatic andundesirable negative effect on the reproducibility of the polishingprocess. The colloidal stability of low-solids abrasive dispersions canbe negatively impacted by a variety of process parameters andphysico-chemical characteristics of the polishing composition, such aspH, contamination with CMP by-products (e.g., in recycled polishingcompositions), the configuration of the slurry delivery system, theslurry delivery flow rate, the residence time of the slurry inun-agitated portions of the delivery system, and the like.

There is an ongoing need to develop new CMP methods and apparatus forutilizing relatively low-solids CMP slurries with a reduced level of TSSvariability and reduced polishing variability. The present inventionprovides such improved CMP methods and apparatus. These and otheradvantages of the invention, as well as additional inventive features,will be apparent from the description of the invention provided herein.

SUMMARY OF THE INVENTION

The present invention provides a chemical-mechanical polishing (CMP)method for polishing a substrate comprising, contacting a surface of thesubstrate with an aqueous slurry comprising a low level of a suspendedparticulate abrasive material, monitoring the TSS level in the slurry atone or more points in a CMP process using a suspended solids sensor, andmaintaining a predetermined TSS level in the slurry by adjusting thelevel of the suspended particulate abrasive material in the slurry basedupon the monitoring by the suspended solids sensor. The CMP methods ofthe invention comprise polishing a substrate with a CMP slurrycontaining a low level of a suspended particulate abrasive material(e.g., about 0.01 percent by weight to about 1.0 percent by weight) in aCMP apparatus, while continuously maintaining a predetermined TSS levelin the CMP slurry by monitoring the TSS level at one or more points inthe CMP process using a suspended solids sensor. Preferably the slurryhas a maximum TSS variability of less than about 20 percent (i.e., ±20%of the target TSS level), preferably less than about 10 percent TSSvariability (i.e., ±10% of the target TSS level) during the course ofthe polishing process.

When the TSS level of the low level abrasive slurry is maintained withina tight variance, the performance characteristics of the substratepolishing process are desirably enhanced. For example, the variabilityin substrate removal rate and/or defectivity desirably can be reduceddue to the reduced variability in TSS level. This is particularlypronounced in slurries containing low levels of suspended particulateabrasive material. The tight process control provided by the methods andapparatus of the present invention can lead, for example, to improvedpolishing productivity, reduced levels of reworking, more predictablesubstrate throughput, and reduced production costs relative toconventional polishing methods without tight TSS control.

The tight control over the TSS level of the slurry used in the polishingmethods of the invention can be achieved by monitoring the TSS level ofthe slurry during the polishing process and adjusting the slurry TSSlevel when it deviates beyond a predetermined amount from thepredetermined target TSS value. The slurry TSS level can be adjusted bychanging the ratio of slurry concentrate and diluent used to make up theslurry. In some cases the slurry TSS level may be adversely affected byinsufficient mixing or agitation in the slurry delivery system, in whichcase the TSS level may be adjusted by changing the mixing parameters orconfiguration of the slurry delivery system.

The present invention also provides a CMP apparatus comprising a movableplaten adapted to hold a polishing pad on the platen, and a movablecarrier assembly adapted to hold a substrate and to urge a surface ofthe substrate against the polishing pad; the apparatus also including aslurry delivery system adapted to contact an aqueous slurry comprising alow level of a suspended particulate abrasive material with the surfaceof the substrate; wherein during use the platen and carrier assembly aredisposed in an opposed, parallel relation to one another with the padand substrate therebetween; the delivery system being adapted to depositat least a portion of the slurry between the pad and the surface of thesubstrate; the CMP slurry delivery system including at least onesuspended solids sensor for measuring the TSS of the slurry. In somepreferred embodiments, CMP apparatus of the invention includes aplurality of TSS sensors within the slurry delivery system, to monitorthe TSS level of the slurry at multiple places within the slurrydelivery system.

The present invention also provides a method of diluting a CMP slurryconcentrate to a target TSS concentration comprising, monitoring the TSSof the slurry in real time using a suspended solids sensor and adding adiluent until the target TSS concentration is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the slurry delivery loop set-up used in the presentevaluations.

FIG. 2 shows a TSS sensor attachment design.

FIG. 3 shows TSS readings from a 6000 ppm standard dispersion of ceria.

FIG. 4 shows TSS readings from a 1500 ppm standard dispersion of ceria.

FIG. 5 shows the results of an iDiel™ 6600 6× dilution test monitored bya TSS sensor.

FIG. 6 shows a plot of Actual Solids vs. Projected Solids for an iDiel™6600 6× dilution test monitored by a TSS Sensor.

FIG. 7 shows 95% confidence intervals obtained from an iDiel™ 6600dilution stage test monitored by a TSS sensor.

FIG. 8 shows box plots of data from an iDiel™ 6600 dilution stage testmonitored by a TSS sensor.

FIG. 9 shows a 6× diluted iDiel™ 6600 TSS sensor repeatability test(Run#1).

FIG. 10 shows a 6× diluted iDiel™ 6600 TSS sensor repeatability test(Run#2).

DETAILED DESCRIPTION A PREFERRED EMBODIMENT

Particulate abrasives useful in the CMP methods of the invention includeany abrasive material suitable for use in CMP of semiconductormaterials. Non-limiting examples of suitable abrasive materials includemetal oxides such as silica (e.g., fumed silica and/or colloidalsilica), alumina, titania, ceria, zirconia, or a combination of two ormore of the foregoing abrasives, which are well known in the CMP art. Apreferred abrasive comprises ceria (cerium oxide). The abrasive materialpreferably is present in the CMP slurry in an amount of not more thanabout 1 percent by weight (10,000 ppm). Preferably, the abrasivematerial is present in the CMP composition in an amount in the range ofabout 0.01 to about 1 percent by weight, more preferably in the range ofabout 0.05 to about 0.6 percent by weight.

The CMP methods of the present invention can be used to polish anysuitable substrate, and are especially useful for polishing substratescomprising silicon dioxide (e.g., PETEOS).

The CMP methods of the present invention are particularly suited for usein conjunction with a chemical-mechanical polishing apparatus.Typically, the CMP apparatus comprises a platen, which, when in use, isin motion and has a velocity that results from orbital, linear, and/orcircular motion. A polishing pad is mounted on the platen and moves withthe platen. A carrier assembly holds a substrate to be polished incontact with the pad and moves relative to the surface of the polishingpad, while urging the substrate against the pad at a selected pressure(down force) to aid in abrading the surface of the substrate. A CMPslurry is pumped onto the polishing pad to aid in the polishing process.The polishing of the substrate is accomplished by the combined abrasiveaction of the moving polishing pad and the CMP composition of theinvention present on the polishing pad, which abrades at least a portionof the surface of the substrate, and thereby polishes the surface.

The methods and apparatus of the present invention can utilize anysuitable polishing pad (e.g., polishing surface). Non-limiting examplesof suitable polishing pads include woven and non-woven polishing pads,which can include fixed abrasives, if desired. Moreover, suitablepolishing pads can comprise any suitable polymer of varying density,hardness, thickness, compressibility, ability to rebound uponcompression, and compression modulus. Suitable polymers include, forexample, polyvinylchloride, polyvinylfluoride, nylon, fluorocarbon,polycarbonate, polyester, polyacrylate, polyether, polyethylene,polyamide, polyurethane, polystyrene, polypropylene, coformed productsthereof, and mixtures thereof.

Desirably, the CMP apparatus further comprises an in situ polishingendpoint detection system, many of which are known in the art.Techniques for inspecting and monitoring the polishing process byanalyzing light or other radiation reflected from a surface of theworkpiece are known in the art. Such methods are described, for example,in U.S. Pat. No. 5,196,353 to Sandhu et al., U.S. Pat. No. 5,433,651 toLustig et al., U.S. Pat. No. 5,949,927 to Tang, and U.S. Pat. No.5,964,643 to Birang et al. Desirably, the inspection or monitoring ofthe progress of the polishing process with respect to a workpiece beingpolished enables the determination of the polishing end-point, i.e., thedetermination of when to terminate the polishing process with respect toa particular workpiece.

A key component of a CMP apparatus of the invention is a TSS monitoringdevice, which comprises at least one suspended solids sensor capable ofproviding an output signal to a recording and/or display device. Theoutput signal is proportional to the TSS of the abrasive slurry. Thesensor can be permanently mounted in the slurry delivery system forcontinuous, periodic, or intermittent monitoring or can be a “hand-held”meter which is intermittently placed into the slurry stream, and whichmay also be used to measure the TSS of samples withdrawn from the slurrydelivery system.

Suitable recording and display devices for receiving and displaying theoutput signal of the TSS sensor include analog and digital devices.Non-limiting examples of analog devices include plotters, strip charts,cathode ray tubes, and the like. Digital recording devices typicallyinclude a microprocessor for receiving the output signal and forconverting the signal into a human or machine readable format, andpreferably include a display device and/or a data storage device.

Adjustment of the TSS level can be performed automatically, in directresponse to the output signal of the sensor or in response to a feedbacksignal from the recording device. Alternatively, the TSS level of theslurry can be adjusted manually based on the output signal or suitableoutput from the recording device.

The following example further illustrates a non-limiting example of aTSS sensor suitable for use in the methods and apparatus of the presentinvention.

Example 1 Evaluation of Total Suspended Solids (TSS) Sensor with iDiel™6600 Dilution

This example illustrates a commercial TSS meter sold under the tradename COSMOS-25 XL, available from Rosemont Analytical Inc. (LaHabra,Calif.).

The purpose of this test was to evaluate the feasibility of monitoringthe dilution of a commercial slurry concentrate, iDiel™ 6600 anddetecting the suspended solids concentration using an in-line totalsuspended solids (TSS) meter.

A dilution evaluation was performed to verify the responses of in-lineTSS readings as parts-per-million (ppm) with step-down addition ofvarious volumes of a DI water diluent. The TSS data was collected during14 different dilution stages. Significant observations made during thisevaluation include the observation that the tested TSS sensor had anaccuracy range for TSS in the range of about 0.1% to about 6.1% over theTSS range of 0.1 percent by weight to about 0.6 5 percent by weightsolids. The variability in repeated tests was less than about 1%. Bycontrast, the standard muffle furnace method for measuring weightpercent solids has a repeatability variation of about 36% for lowsuspended solids slurry measurements.

The slurry distribution loop set-up used in these tests is shown inFIG. 1. The loop includes about 10 to 20 feet of ¾ inch tubing; anin-line TSS meter (Rosemount Analytical Züllig COSMOS-25 XLsensor/b-line V transmitter); a 55 inch TSS attachment mounted inupright position with an upwards flow path (316 stainless steel ¾ inchFNPT connections for the flow path, sanitary connection for TSS sensor;see FIG. 2); and a 30-gallon container. The slurry was re-circulated inthe loop with a bellows pump.

TSS sensor calibration was performed using the calibration procedurefrom the manufacturer's operations manual. Calibration of the TSS sensorfor this application used two (2) standard dispersions of iDiel™ 6600 at6,000 ppm (0.6% solids) and at 1,500 ppm (0.15% solids; made by diluting2,500 g iDiel™ 6600 with 7,500 g DI water (1:3). The TSS readings forthe 1st and 2nd calibration dispersion can be seen in FIGS. 3 and 4,respectively.

A drum of iDiel™ 6600 was mixed with an A310 impeller mixer for about anhour at about 1,725 rpm and about 18,000 g of iDiel™ 6600 wastransferred into the 30-gallon container.

The bellows pump was started and was run continuously for about 3 hoursat about 5 gallons-per-minute (gpm). DI water was added to the slurry inthe 30 gallon container at various stages. Details of the amounts of DIwater added are provided in Table 1.

TABLE 1 Dilution of iDiel ™6600. % of total DI % total DI water Stage DIwater added water weight accumulation 1 305 0.34% 0.34% 2 5000 5.56%5.89% 3 5000 5.56% 11.45% 4 5000 5.56% 17.01% 5 5000 5.56% 22.56% 6 50005.56% 28.12% 7 5000 5.56% 33.67% 8 5000 5.56% 39.23% 9 10000 11.11%50.34% 10 10000 11.11% 61.45% 11 10000 11.11% 72.56% 12 6695 7.44%80.00% 13 8181 9.09% 89.09% 14 9819 10.91% 100.00%

Data acquisition was performed throughout the testing period to recordTSS as ppm. After Stage 14, the final DI water dilution was equivalentto 5 parts of DI water to 1 part of slurry, and the original 0.6 percentby weight solids of the concentrated slurry was diluted to about 0.1percent by weight. The final total weight of diluted slurry was 108,000g (90,000 g of DI water and 18,000 g of iDiel™ 6600).

A first repeatability test was then performed. After completing theDilution Test (described above), the slurry from Stage 14 wascontinuously recirculated in the loop for about 115 hours (about 4.8days) to test the repeatability of the TSS sensor. After completing thisfirst repeatability test, all slurry was drained from the loop/pumpsystem. The system was then flushed with DI water and drained. A secondrepeatability test was then performed. A new diluted (1:5) batch ofiDiel™ 6600 (2,900 g dispersion and 14,500 g DI water) was made for thissecond test. All the setup and process parameters were the same as forthe first repeatability test, except that a 5 hour run time was used.The results of the tests are shown in FIGS. 5-10 and Table 2.

TABLE 2 Projected solids concentration vs. mean actual solidsconcentration recorded from TSS sensor and data acquisition system.Projected TSS (ppm) Mean Observed TSS (ppm) % error 6000 5974 0.4% 59005906 −0.1% 4634 4780 −3.1% 3816 3981 −4.3% 3243 3335 −2.8% 2819 2884−2.3% 2494 2531 −1.5% 2236 2246 −0.5% 2026 2015 0.5% 1706 1680 1.5% 14731425 3.3% 1296 1242 4.2% 1200 1141 4.9% 1100 1040 5.5% 1000 939 6.1%

From the TSS repeatability test, the mean solids concentration for the115 hour first repeatability test (see FIG. 9) and the 5 hour secondrepeatability test (see FIG. 10) were 937 ppm and 953 ppm, respectively.The theoretical target suspended solids concentration was 1,000 ppm. Therespective TSS variations for test 1 and test 2 were about 6.7% andabout 4.7% off from the target TSS. However, the measurements wererepeatable within 95% confidence interval (CI) in both test runs. Thedifference in mean TSS between these two tests is probably due todifference in the dilution methods. For example, test 1 used a 14-stagedilution and test 2 used a single-stage dilution.

According to the data collected during each dilution stage, thevariability in repeated TSS measurements was less than about 1% duringshort term (less than 5 minutes in FIGS. 3 and 4) and long term (about 4days in FIGS. 9 and 10) testing. The 6000 ppm and 1500 ppm calibrationslurries were read as 5994 ppm (FIG. 3) and 1485 ppm (FIG. 4),respectively, and indicate variation for each slurry of 1% or less.

The accuracy of the TSS sensor ranges from about 0.1% to about 6.1%error during the various dilution stages (see Table 2). The actual TSSvs. projected TSS had a good fit on linearity and accuracy (R²=0.9986,FIG. 6).

From the data shown in FIGS. 7 and 8, it is clear that the variation and95% CI were very tight at each dilution stage. The number of data pointscollected at each stage varied from 54 to 511. Therefore, a close andaccurate monitoring of slurries having a low level of suspended solidsis possible using the methods and apparatus described in the presentinvention.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

1. A chemical-mechanical polishing (CMP) method for polishing asubstrate comprising: (a) contacting a surface of the substrate with anaqueous slurry comprising a low level of a suspended particulateabrasive material; (b) monitoring the TSS level in the slurry at one ormore points in a CMP process using a suspended solids sensor; (c)maintaining a predetermined TSS level in the slurry by adjusting thelevel of the suspended particulate abrasive material in the slurry basedupon the monitoring by the suspended solids sensor.
 2. The method ofclaim 1 wherein the TSS level of the slurry is in the range of about0.01 percent by weight to about 1.0 percent by weight.
 3. The method ofclaim 1 wherein the TSS level of the slurry is in the range of about0.05 percent by weight to about 0.6 percent by weight.
 4. The method ofclaim 1 wherein the particulate abrasive material comprises a metaloxide.
 5. The method of claim 4 wherein the metal oxide is cerium oxide.6. The method of claim 1 wherein the TSS of the CMP slurry has a maximumTSS variability during the CMP process of less than about 20%.
 7. Themethod of claim 1 wherein the TSS of the CMP slurry has a maximum TSSvariability during the CMP process of less than about 10%.
 8. A CMPapparatus comprising a movable platen adapted to hold a polishing pad onthe platen, and a movable carrier assembly adapted to hold a substrateand to urge a surface of the substrate against the polishing pad; theapparatus also including a slurry delivery system adapted to contact anaqueous slurry comprising a low level of a suspended particulateabrasive material with the surface of the substrate; wherein during usethe platen and carrier assembly are disposed in an opposed, parallelrelation to one another with the pad and substrate therebetween; thedelivery system being adapted to deposit at least a portion of theslurry between the pad and the surface of the substrate; the CMP slurrydelivery system including at least one suspended solids sensor formeasuring the TSS of the slurry.
 9. The apparatus of claim 8 wherein theat least one suspended solids sensor comprises an in-line TSS monitoringdevice disposed within the slurry delivery system near an outlet fordepositing the slurry onto the polishing pad.
 10. A method of diluting aCMP slurry concentrate to a target TSS concentration comprising,monitoring the TSS of the slurry in real time using a suspended solidssensor and adding a diluents until the target TSS concentration isachieved.
 11. The method of claim 10 wherein the target TSSconcentration of the CMP slurry is in the range of about 0.01 percent byweight to about 1.0 percent by weight.
 12. The method of claim 10wherein the target TSS concentration of the CMP slurry is in the rangeof about 0.05 percent by weight to about 0.6 percent by weight.
 13. Themethod of claim 10 wherein the particulate abrasive material comprises ametal oxide.
 14. The method of claim 13 wherein the metal oxide iscerium oxide.